1. Field of the Invention
The present invention relates to a lightning arrester for instantaneously protecting an electric circuit from abnormal voltage.
2. Description of the Prior Arts
Heretofore, it has been known to use the device shown in FIG. 1 as the protective device.
In FIG. 1, the reference numeral (1) designates an earthing tank which has an inner part (2) as a space filled with a gas having high insulating strength such as SF.sub.6 ; (3) designates a resistor having excellent non-linear characteristic, for example a resistor formed by superposing sintered substrates having main component of zinc oxide; (4) designates a conductive substrate as a lead wire in high voltage side; (5) designates an insulating spacer for holding the lead wire (4).
The operation of the conventional protective device will be illustrated.
The lead wire (4) is connected to the terminal for high voltage of the apparatus which should be protected, and the surge given by thunderbolt etc. is shorted through the resistors (3).
FIG. 2 shows one example of voltage-current characteristic of the zinc oxide sintered element used as the resistor (3). The raising of the terminal voltage to the surge voltage is lowered by the constant voltage characteristic in a wide range.
The full line in FIG. 2 shows the characteristic to DC or large current surge.
The voltage-current characteristics in the case of applying AC voltage to said element as the peak values of the current and voltage, are different from those of the DC voltage in the small current region as shown by the broken line of FIG. 2, since the element has electrostatic capacity. These characteristics are common to the zinc oxide sintered element and the other non-linear resistors.
The voltage-current characteristics for relatively high AC voltage are similar to those of the DC voltage.
In FIG. 2, the characteristic curves for AC and DC voltages are substantially similar when the voltage is higher than Vo. However, they are different when the voltage is lower than Vo.
The current at Vo is usually higher than 1 mA in the case of zinc oxide sintered element.
Thus, AC line voltage is normally applied to the non-linear resistor in the AC lightning arrester.
The normal earthing voltage should be the level being lower than Vo such as Vp shown in FIG. 2 from the viewpoint of the life of the element as described below.
The following troubles are caused because these elements function as a simple capacitor at low AC voltage.
In the structure of FIG. 1, the floating capacity is formed between the non-linear element (3) and the tank (1) whereby the voltage allotment of the non-linear resistor by the equivalent circuit as shown in FIG. 3 to the low AC voltage such as the normal earthing voltage should be considered.
In FIG. 3, the reference H designates a total length of the non-linear resistor; X designates a distance from the high voltage terminal to the point considered; d.sub.X designates a differential distance for the following calculation; K/d.sub.X designates an electrostatic capacity of the element at the part of d.sub.X ; Cd.sub.X designates an electrostatic capacity between the part of d.sub.X and the tank; and V.sub.X designates an electric potential of the non-linear resistor at the point X to the voltage V.
The following equation is given ##EQU1##
When the references C and K are constant regardless of X, the equation can be as follows. ##EQU2##
The equation is considered in the boundary condition V.sub.(o) =V ##EQU3##
The potential distribution V(X) of the non-linear resistor is given in the form shown by the full line in FIG. 4(a), which is different from the linear potential distribution shown by the broken line.
As it is clear from the above-mentioned equations, the deviation from the linear potential distribution is increased depending upon the increase of the length of the non-linear resistor.
As the result, the field E(X)=.vertline.dV(X)/dX.vertline. in the inner part of the non-linear resistor is significantly non-uniform as shown by the full line in FIG. 4(b).
The maximum field is formed in the high voltage side (X=0), and the field Emax at the maximum field point is significantly higher than the average field Eav.
In this circumstance, it is found the overvoltage to the level of the normal earthing voltage Vp shown in FIG. 2 at the part near the high voltage side of the non-linear resistor.
When the overvoltage is normally applied to the element, the element is electrically deteriorated.
FIG. 5 shows one example of the curve of voltage-life of the zinc oxide element.
The life is significantly shorter depending upon approaching the voltage to Vo.
Accordingly, in the conventional structure, the normal earthing voltage is deviated to the high voltage side and the non-linear resistor at the part, significantly deteriorated, disadvantageously.